Are there any design rules/regs for rainwater soakaways for a single dwelling

I take it this is your own place C? As per the above link the efficiency of a soakaway is entirely prevalent on a percolation test of the surrounding soil. If the water percolates from the trial pit sufficiently into the surrounding soil a soakaway is perfectly feasible but pointless seeing as the soil will drain it anyway. If the surrounding soil won't drain it sufficiently then the soakaway is pointless as it will just fill up and overflow.